To get an information about how long the execution of a cronjob takes it would be a good idea to add timestamps to runcronjobs.php, e.g.

+if ( !$isQuiet )
+ $cli->notice( "+++ Start: " . date("m.d.y G:i:s T") );

$ini =& eZINI::instance( 'cronjob.ini' );

...

+if ( !$isQuiet )
+ $cli->notice( "+++ Stop: " . date("m.d.y G:i:s T") );
+
$script->shutdown();